Living organisms are autonomous chemical systems which include diverse sets of small molecules. Small molecules found in living systems include, for example, sugars, fatty acids, amino acids, nucleotides, and intermediates of metabolic and signaling pathways. Sugars are a primary source of chemical energy for cells. The cells break the sugars down through a series of oxidative reactions to small sugar derivatives and, ultimately, CO2 and H2O. Fatty acids used for both energy storage and as major components of cellular membranes. Amino acids are the building blocks of proteins. Nucleotides are involved in intracellular signaling, energy transfer, and as the monomers of the information macromolecules, RNA and DNA.
The cellular small molecules are, generally, composed of six elements (C, H, N, O, P, S). If water is excluded, carbon compounds comprise a large majority of the cellular small molecules. The cellular small molecules repeatedly use certain distinctive chemical groups, such as methyl (CH3), carboxyl (COOH) and amino (NH2) groups.
Generally, most cellular small molecules are synthesized from and broken down to the same basic compounds. Synthesis and metabolism occurs through sequences of controlled chemical reactions, catalyzed by enzymes. Most of the metabolic reactions of the cell occur in the cytoplasm, which contains many distinctive organelles. For example, the mitochondria are responsible for respiration and energy production. Mitochondria are the “power plants” of eukaryotic cells, harnessing energy contained by combining oxygen with metabolites to make ATP. Other organelles of the cell include the Golgi apparatus, a system of stacked, membrane bound, flattened sacs involved in modifying, sorting and packaging of macromolecules for secretion or for delivery to other organelles. The endoplasmic reticulum (ER) is a series of flattened sheets, sacks, and tubes of membrane extending throughout the cytoplasm of eukaryotic cells. The ER membrane is in structural continuity with the outer membrane of the nuclear envelope and specializes in the synthesis and transport of lipids and membrane proteins.